The present invention relates in general to perimeter monitoring and intrusion detection, and in particular to the detection of intruders by the interruption of beams traversing between a millimeter wave radar and associated apparatus.
The possibility of industrial espionage, terrorist activity or material theft result in the necessity to protect various high value assets, whether they are owned by the military, government, utility companies, corporations or private entities. High value assets are frequently protected by wire fences or solid walls. However, these means of protection can be surmounted or penetrated by a determined intruder. An example of an asset that may need protection is an aircraft away from its home base and parked on the tarmac with only airport perimeter fencing for security.
A desirable method of protecting valuable assets is the establishment of an xe2x80x9celectronic fencexe2x80x9d that includes a volume or volumes wherein any intruder entering any of these volumes will be detected. These volumes should have clearly defined limits to prevent false alarms due to individuals, vehicles, and so on, transmitting nearby but not through the detection volume. FIG. 1 illustrates such an electronic fence arrangement established to protect a high value asset 1. The high value asset is shown parked in an open storage area formed by buildings 2 and a perimeter fence 3.
When this level of security is deemed insufficient, an electronic fence can be formed by the present invention to provide the needed detection of any intruder that has gotten inside the perimeter fence. In the arrangement of FIG. 1, the electronic fence has segments 4 and 5. If the asset should be located in an open area such as the aircraft example above, the fence can be deployed with a sufficient number of segments to completely surround the asset.
Prior art use of fences for intruder detection have employed micrometer, millimeter or infrared wavelengths in the electromagnetic spectrum. Various methods of generating beams or electronic fences as well as methods of detection have been used.
Prior art infrared intrusion detectors use such means as the formation of a beam or beams traversing the detection area with the breaking of the beam by the passage of an intruder initiating a detection, or the reflection of the beam off the intruder back to a receiver as a means of detection. Still another prior art method comprises a at passive system that detects the difference in temperature between the ambient environment and the intruder. The usefulness of the infrared class of prior art is limited by atmospheric conditions including heavy rain and fog that may interrupt a beam, and by the growing availability of infrared viewing equipment that may make infrared beams visible to a well-equipped intruder.
Examples of prior art operating in the microwave and millimeter wave region are capable of generating confined beams to generate an electronic fence. Some place a transmitter at one end of the fence volume and a receiver at the other to form a beam or detection volume between the two. The patent Blacksmith, et al., U.S. Pat. No. 4,132,988, issued on Jan. 2, 1979, uses this configuration with a plurality of passive reflectors to surround a rectangular area and to place the receiver near the transmitter. The passive reflectors are used to change the direction of propagation of the beam, typically by 90 degrees, several times to surround the high value asset and return the beam to the transmitter/receiver location. Interruption, or breaking, of the beam initiates an alarm. In this and similar systems a single beam is used with a cross section defined by the directivity of the antennas, and dimensions of the passive reflectors. An astute intruder could devise means to go under or over the beam and thus avoid detection.
Other prior art sensors based on radar concepts generate beams confined in azimuth and elevation and seek to detect an intruder within the thusly-defined fence volume by ranging on the radar return from the intruder. These sensors require significantly greater emitted energy than beam breaker systems.
What is needed is an improved intrusion detection method and apparatus that generates complete coverage from the surface up to a selected height and along the entire length of the electronic fence to greatly minimize the probability of an intruder traversing the fence by proceeding without being detected over or under the beam or beams that comprise the fence. The intrusion detection apparatus should be easily transportable, consume a minimum of prime power, emit a minimum of electromagnetic energy, have a high probability of detection of any intruder traversing the electronic fence, and have a low false alarm rate for any entity near the fence. For these and other reasons, there is a need for the present invention.
The invention relates to an electronic fence that uses high-resolution millimeter-wave radar in conjunction with multiple passive reflectors. The invention provides a new and improved method for the detection of any intruder attempting to traverse a volume defined as an electronic fence, and the apparatus for the implementation of this fence. One advantage of the present invention is that both radar returns from intruders and a plurality of electromagnetic beams operating in the millimeter wave region of the spectrum are used to generate the electronic fence. At these wavelengths emitted energy is confined to well defined beams formed by reasonably sized antennas.
Another advantage of the invention is that the radar reflection is used to report the location of any intrusion for an intruder attempting to traverse the electronic fence near the sensor end, while breaking of one or more electromagnetic beams constitutes the primary means of detection at greater ranges. However, the radar reflections from most intruders at greater ranges are detected and used to report the distance from the sensor structure to the intruder location. A further advantage of the invention is that a sufficient number of beams inhabit the fence volume from a point a short distance away from the sensor end to the terminus end to make traversing the fence by an intruder without detection through this region essentially impossible. This is even if the intruder might attempt such detection avoiding methods as crawling under or jumping over the assumed location of the beams. Still another advantage of the invention is that the continuity of each individual beam is monitored at a sufficiently high rate that an intruder cannot traverse any beam or beams without being detected by beam interruption.
A further advantage of the present invention is that multiple beams are formed by emitting and receiving antennas included in a sensor structure located at one end of the electronic fence. A multiplicity of passive retroreflectors of one form is located periodically along the base of the electronic fence volume and of another form within the reflector structure forming the terminus of the fence. Each antenna functions as both an emitter and a collector of electromagnetic energy within its design beamwidth. The millimeter wave energy proceeding from an antenna to a retroreflector and returning to the antenna along substantially the same path constitutes a beam. The passive retroreflectors are each located at a different actual or apparent range from the emitting and antennas in the sensor structure. A receiving apparatus coupled to the antennas includes means to assess the amplitude of the returned signal from each range separated passive retroreflector.
Another advantage of the invention is that the reflector structure contains a vertically stacked plurality of passive retroreflecting elements, each formed by an antenna coupled to a shorted waveguide of length different from that of the other waveguides. Each antenna and shorted waveguide assembly constitutes a retroreflector at a different apparent range, allowing the sensor structure receiving apparatus to differentiate between the returns from each antenna. This differentiates between the different beams arriving at different locations from base to top of the reflector structure. Other advantages are that all energy-consuming elements are included within the sensor housing, power consumption is minimal due to beam formation by highly efficient passive retroreflectors, and all components are easily transportable and can be deployed rapidly. Still other aspects, embodiment, and advantages of the invention will become apparent by reading the detailed description that follows, and by referencing the accompanying drawings.